1. Field of the Invention
The invention relates to a process for electrolytic coating of a strand casting mould.
2. Description of Related Art
Strand casting moulds are subject to a constant abrasive wear during casting, so that the mould cavity and therefore also the cross-section dimensions of the cast strands become ever larger. After a certain number of working cycles, the particular strand casting mould must therefore be replaced by a new one or reworked.
Various methods for reworking the moulds for the purpose or re-establishing the original geometry of the mould cavity or the intended dimensions of the mould cavity are known. Reworking can be carried out, for example, by explosion forming of the mould on a mandrel. Not only is this method relatively complicated, expensive and environment-polluting, it also means a deformation of the external shape of the mould, which in turn involves an enlargement of a water gap present on the periphery of the mould and as a result an adverse influence on the cooling of the mould. Other known pressing processes for reshaping the moulds in which the mould is first compressed from the outside and the mould cavity is then brought to the original internal dimensions by internal grinding or internal milling also have the latter disadvantage.
Finally, it is known from EP-A-0 282 759 to bring the mould cavity of a strand casting mould back to the intended dimensions by electrolytic coating of the internal surfaces which demarcate the mould cavity. In this generic process, the mould, which serves as the cathode, is immersed in an electrolyte bath (Cu sulfate bath) together with a perforated anode basket which is positioned in the mould cavity and is filled with soluble copper pieces (cubes, balls, discs). When a direct current is connected, the copper is separated out of the electrolyte bath and deposited on the mould surfaces, the copper separated out of the electrolyte bath being replaced by the dissolved anode copper. A relatively low current density, for example of about 15 A/dm2, is achieved in this dipping electrolytic process. From experience, in the case of electrolytic dip-coating of mould cavities which are usually polygonal in cross-section there is the risk that the layer is of insufficient thickness in the corner regions, that is to say the layer thickness is only about ¼ to 1/10 of that in the other regions. This non-uniform layer build-up can be only partly remedied with special anode geometries. This means a further mechanical reworking is necessary.
With the production of thick layers there is furthermore the risk that corner bridges with enclosed cavities are formed, as a result of which the mould becomes unusable. A further disadvantage of electrolytic dip-coating is that the external surfaces of the mould must be covered with a material which is inert towards the electrolytic treatment.